In semiconductor manufacturing and other fields it is desirable to make quantitative measurements of a sample's reflectivity properties over very small, selectable areas and over as broad a band of wavelengths as possible. Instruments for making these measurements typically incorporate microscope-like objective lenses for focusing light on the sample. One type of illumination source is a Xenon arc lamp. In order to scan to various positions on the sample of interest a portion of the optics can be moved with respect to a stationary portion of the optics, or the sample can be moved with respect to at least some portion of the optics, or both.
A common issue with such instruments is guaranteeing the stability of the light which is incident upon the sample, or at least knowing the spectral intensity of the incident light, so that the detected light reflected from the sample can be evaluated relative to the intensity of the light incident on the sample. Since reflectivity is defined as the ratio of the intensity of light reflected off the sample relative to the intensity of light incident upon the sample, accurate reflectivity measurements depend knowing the incident light intensity.
There are several factors that can make it difficult to determine the intensity of the light upon the sample. One factor is that the characteristics of most sources of light change with time, and thus the intensity of the incident light can vary with time. Another factor is that where there is relative motion of the illumination source and the rest of the optics, either via (nearly) collimated light paths or optical fibers, there can be changes in the transmission efficiency of illuminating light as a function the scan position, or scan state. Here scan state includes the history of previous scan positions. This is important, for example, with some architectures using a fiber to transmit light from the light source. One prior system is shown in U.S. Pat. No. 6,667,805 (SMALL SPOT SPECTROMETRY INSTRUMENT WITH REDUCED POLARIZATION) and in PCT application, international publication number WO 00/57127 (METHOD AND APPARATUS FOR WAFER METROLOGY); both of these references are incorporated herein by reference in their entirety.
FIG. 1 shows another type of prior system 100. The system 100 includes a light source 102 and a transmission means 104 for light generated by the light source 102. The light transmitted through the transmission means 104 is then transmitted through a collimating lens 108, and leaves the collimating lens as light beam 106. The light beam is then incident upon a beam splitter 110. A first beam 140 is transmitted from the beam splitter through a lens 144 and then through a plate 146 having pinhole to receive the first beam 140. The first beam 140 is then transmitted through a transmission means 148 and received by a detector 150. In response to the light received, the detector 150 generates a monitor signal corresponding to the received light. This monitor signal from the detector 150 is received by a processor 160 which analyzes the monitor signal and uses it relative to a signal generated by detector 130.
In addition to the beam 140 being transmitted through the beam splitter 110, beam 112 is also reflected from the beam splitter 110 through an objective lens 114 and onto a spot 118 on a sample 116 being analyzed. Some portion 113 of the light 112 is reflected off the sample and back through the objective lens 114. This light 113 is further transmitted through the beam splitter 110 and off a turn mirror 122 and through a lens 124. The resulting light beam is then transmitted through a pinhole in a plate 126 and into a transmission means 128. The light transmitted through the transmission means 128 is received by the detector 130. In response to receiving this light the detector 130 generates a sample signal which corresponds to the received light. This sample signal is received by the processor 160 where it is analyzed relative to the monitor signal received from the detector 150.
The fact that prior systems provide for transmitting a monitor beam 140 and a measurement beam 113 through different transmission paths and provide for using different detectors (150 and 130) for detecting the monitor light beam and the measurement light beam introduces a number of potential sources which could generate variations in the monitor signal relative to the measurement signal which are not related to the reflective properties of the sample. What is needed is a system which reduces possible sources of extrinsic variations in the monitor beam relative to the measurement beam.